1. Field of Invention
This invention relates to a method and apparatus that measure material thickness using an ultrasonic wave, and particularly to a method and apparatus that are suitable for measuring material thickness in an environment in which temperature of the material significantly changes.
2. Description of Related Art
As a material thickness measurement method, a method is well known in which an ultrasonic wave is transmitted in a material thickness direction and the material thickness is measured from the product of the time of flight and the ultrasonic wave velocity of the material. Many thickness measuring devices using this method are sold in the market. FIG. 1 schematically shows a material thickness measurement principle according to this method. Hereafter, a material thickness measurement method using an ultrasonic wave is explained based on FIG. 1.
FIG. 1 shows a conventional thickness measuring device 100. An ultrasonic wave probe 110 constituted by a piezoelectric transducer or the like contacts the surface of a material M, with glycerol or the like as a couplant, and an ultrasonic longitudinal wave, produced by an ultrasonic wave transmitter 120, is generated inside the material M via this ultrasonic wave probe 110. This longitudinal wave is reflected by a bottom surface of the material M, and returns to the incident surface of the material M that is contacted by the probe. When the longitudinal wave returns to the incident surface, the incident surface has a transient displacement that is proportional to the amplitude of the arriving longitudinal wave. This displacement of the surface of material M is received by the same ultrasonic wave probe 110 as is used for ultrasonic wave generation.
FIG. 2 schematically shows the received waveform at this time. T shows the transmitted pulse, and 2L and 4L show first and second bottom surface echoes which have echoed once and twice, respectively, within the material M. An arrival time interval t2L of the pulses T and 2L is measured by a longitudinal wave time of flight measuring device 130. A calculator 140 calculates the material thickness D by using this arrival time interval t2L, and an ultrasonic longitudinal wave acoustic velocity VL of the material which was obtained in advance. The material thickness D is obtained by the following equation (1).D=VL·t2L/2  (1)
Alternatively, the material thickness D can be calculated by measuring the arrival time interval t2L′ of the pulses 2L and 4L by the longitudinal wave time of flight measuring device 130, and using the following equation (2).D=VL·t2L′/2  (2)
In this type of material thickness measurement method, the piezoelectric transducer 110 has to contact the measured material M. Therefore, when the measured material M has a high temperature or moves at a high speed, e.g., such as in a metal manufacturing line, application of this method is difficult. Furthermore, the velocity VL of the ultrasonic wave significantly changes depending on the material temperature. Thus, in order to measure the material thickness D with high accuracy, the ultrasonic wave velocity at the time of measurement needs to be accurately obtained by separately detecting the temperature of the material M.
As a method of solving this type of problem, a method using a laser ultrasonic technique and a pyrometer is described in “Proceedings of the 39th Mechanical Work Steel Process Conference”, ISS, Vol. XXXV, p.927 (1998), incorporated herein by reference in its entirety. In this method, a high energy pulsed laser beam is irradiated onto a material surface, and ablation is generated in the material surface. An ultrasonic longitudinal wave is transmitted inside the material by the reaction force of the ablation. This longitudinal wave travels through the material and is reflected by the bottom surface of the material. A transient displacement of the material surface is generated when the longitudinal wave returns to the incident surface, and this displacement is detected in a non-contact manner by using an optical interferometer. Separately from the ultrasonic device, a pyrometer is used to monitor the material surface temperature. A correlation of longitudinal wave velocity and material temperature is obtained in advance, and effects due to the change of the material temperature are corrected based on this correlation.
Furthermore, Japanese Laid-Open Patent Application 54-97447 discloses a method in which a material thickness is obtained using times of flight of shear and longitudinal ultrasonic waves generated by an electromagnetic acoustic transducer. In this method, by using two sets of electromagnetic acoustic transducers, the respective ultrasonic longitudinal and shear waves are generated inside the material and detected, and the ratio of the times of flight of the longitudinal and shear waves is obtained. Thereafter, according to a correlation of the time of flight ratio and material temperature that has been obtained in advance, the material temperature is obtained. From the correlation of the time of flight of the shear or longitudinal wave and the material temperature obtained in advance, a velocity of the shear wave or longitudinal wave is obtained. The material thickness is calculated according to the above-mentioned equation (1) or (2). In this method, there is no need for separately arranging a temperature measuring device such as a pyrometer, and the velocity can be corrected by monitoring a material internal temperature instead of a material surface temperature. Therefore, the material thickness can be measured with higher accuracy than in the above-mentioned method using a laser ultrasonic technique and a pyrometer.
Other laser ultrasonic techniques are disclosed in “Progress Towards the Application of Laser-Ultrasonics In Industry”, Review of Progress in Quantitative Nondestructive Evaluation, Vol. 12, pp.495-506 (1993) and in U.S. Pat. No. 6,078,397 to Monchalin et al., each incorporated herein by reference in its entirety.